And righteousness shall be the girdle of his loins— We have here the basis and foundation of this oeconomy, namely, the justice and fidelity of the king. Girdles were worn by the easterns both for ornament and use.
The metaphor here implies that these virtues of righteousness and fidelity, or truth, are the proper and true virtues of Christ the king; most closely adhering to him, as a girdle cleaveth to the loins of a man, Jeremiah 13:11 that these virtues should be conspicuous in the whole administration of his kingdom; and at once be the ornament and the support of it. The sum is, that the kingdom of Christ should be a kingdom of the highest equity, and the king of it most perfect; who, though judging his true subjects by the law of grace, by faithfully performing all the promises of the Gospel, and every condition of the covenant to them, will yet not omit to punish the enemies of his church according to their deserts, and thus to satisfy the law of justice: so that he shall not be less venerable and awful for his justice in judgment, than amiable and desirable for his truth, fidelity, and constancy in performing his promises; which, being things naturally united, are not by any means to be separated.